<!DOCTYPE html>
<html>
  <head>
    <style>
      p{
        font-size: 20px;
      }
      #box1 .spec2 p {
        color: #f00;
      }
      div div #box3 p {
        color: rgb(0,255,0);
        font-weight: bold;
      }
    </style>
  </head>

  <body>
    <div id="box1" class="spec1">
      <div id="box2" class="spec2">
        <div id="box3" class="spec3">
          <p>段落文字</p>
        </div>
      </div>
    </div>
  </body>
</html>